A magnetic resonance imaging device produces a measurement of a sample which is based upon its molecular structure. The sample is subjected to a polarizing magnetic field which has the effect of aligning the spins of all the atomic nuclei of the sample. Radio waves at a frequency close to the Lamor frequency of the nuclei are then used to excite the nuclei such that their magnetic alignment is reversed. Once the excitation is removed the nuclei return to their original state by emitting characteristic radio signals. It is these radio signals that can be used to image the sample.
The exact Lamor frequency is dependent upon the precise magnetic field. By creating a magnetic field gradient within the sample cavity the source of these signals can be located such that an overall image of the sample can be constructed.
The efficiency of this process depends upon the consistency of the magnetic field strength within the sample cavity. This field is typically controlled to within 5 parts per million. The extent of the uniformity of the magnetic field determines the accuracy with which the Lamor frequency can be measured. This allows for the resolution of smaller chemical shifts.
The magnetic field is dependent to a large extent upon the ferromagnetic properties of the materials making up its magnetic assembly. These ferromagnetic characteristics are often temperature dependent and so it is important to maintain the magnetic assembly at a constant temperature. This can be particularly problematic where the temperature of the sample being imaged is itself fluctuating. In such a scenario it is necessary to introduce a means of insulating the magnetic assembly from the magnetic assembly.
Passive insulation is insufficient to stabilize a sensitive permanent-magnet magnetic resonance system. This is due to the nature of permanent magnet material that has a sensitive temperature coefficient that causes the magnetic field of the system to fluctuate beyond an acceptable range. There is thus a long felt need in the art for a means and method of maintaining a constant temperature in the magnetic assembly of a magnetic resonance device.